1. Technical Field
The present invention relates generally to Q-switched lasers and more particularly to a method and apparatus for the detection and control of prelasing in Q-switched lasers.
2. Discussion of the Related Art
A laser uses the principle of amplification of electromagnetic waves by the stimulated emission of radiation. As shown in FIG. 1, a laser comprises a resonator 12 defining a resonating cavity 14 which contains an appropriate lasing medium M. A pump 16 such as an optical pump excites the lasing medium, stimulating it to emit light. Any conventional excitation method may be employed. A reflecting mirror endplate 18 is located at one end of cavity 14 and a partially transparent, partially reflecting mirror endplate 20 is located at the opposite end. The endplate 20 may be any conventional endplate such as those having silver, metallic or dielectric coatings. Light radiation that is directed along the longitudinal axis of the cavity bounces back and forth between the endplates, further exciting the lasing medium M. The conventional laser fires when the radiation level is sufficient to exit the partially transparent endplate 20.
The output power of pulsed lasers can be greatly increased, with correspondingly shorter pulse duration, by the Q-switch technique. In this method, the optical path between partially transparent endplate 20 and the lasing medium is blocked by a reflective Q-switch shutter 22 controlled by a Q-switch circuit 24. The circuit keeps shutter 22 closed while the lasing medium is excited past the normal lasing level. At a predetermined point, the Q-switch circuit opens the shutter and the stored energy is released in a giant pulse. A good discussion of Q-switches is found in the Background information section of U.S. Pat. No. 4,740,986 to Reeder, the specification of which is hereby incorporated by reference.
In Q-switched pulsed laser systems there is a condition called prelasing which occurs when laser light "leaks" out of the laser cavity prematurely. This leakage is due to the inability of the Q-switch to completely hold off the lasing action of the laser cavity. Due to the optical damage effects associated with prelasing, the condition is considered to be undesirable and needs to be detected and avoided.
The undesirable effects of prelasing fall into two general categories. The first are those directly related to the optical damage effects on the laser system optics and on the optics of the recipient of the laser system output. This optical damage can be quite costly both to the laser manufacturer and the laser user due to both the financial costs associated with the replacement cost of the optics and the system down time necessary to correct the damage. This repair down time is usually quite inconvenient and annoying. The second category includes those problems not related to optical damage effects but to the effects on the applications of a prelasing laser beam. Detrimental effects on the laser output applications would include those associated with multiple output beams, varying energy per laser pulse, changing average output power and output pulsewidth problems.